Rubber articles, such as tires, belts and hoses, normally use reinforcing materials such as steel, polyester, nylon, aramid and rayon in the form of fibers, cords or fabrics. In the case of radial tire production, steel cords are often used as the reinforcing material. Steel cords for tires, in general, are coated with a layer of brass to promote the adhesion between the steel cords and the rubber compositions or compounds (designated hereafter as the steel cord adhesion). It is believed in some instances that the copper and zinc metal in the brass coating may react with sulfur, a vulcanizing agent in some rubber compositions, to form a bonding or sulfide layer, comprising sulphides of copper and zinc, between the steel cord and the rubber compositions. The formation of such a sulfide layer at the interface is responsible for the initial unaged steel cord adhesion.
To improve the steel cord adhesion, the current practices include adding in the rubber compositions a cobalt salt, such as cobalt naphthenate, and/or a phenolic adhesive composition comprising a methylene acceptor and methylene donor. The use of cobalt salt may regulate the formation and the composition of the bonding layer so as to affect the steel cord adhesion. The steel cord adhesion can also be improved by using novolak resins as a methylene acceptor and hexamethoxymethylmelamine (HMMM) or pentamethoxymethylmelamine (PMMM) as a methylene donor in the rubber compositions. On curing the rubber compositions, the reaction product of the methylene acceptor and the methylene donor forms a protective moisture resistant coating on the top of the bonding layer and protects the loss of the steel cord adhesion during aging.
Achieving higher levels of the steel cord adhesion and maintaining the steel cord adhesion under various environmental conditions, such as heat, humidity and saline conditions, are desirable for the long term durability of rubber tires. In an unaged condition, the steel cord adhesion generally exceeds the tear strength of the rubber composition and therefore, no bond failure occurs at the interface between the bonding layer and the rubber composition. However, in wet and salt water conditions, the steel cord adhesion may fail at the interface due to corrosion. Although the cobalt salt may be effective against the corrosive effects of salt water and moisture/steam, the use of novolak resins, along with HMMM, has also provided excellent steel cord adhesion under these corrosive conditions.
It is believed that the corrosion of steel cords is due to the attack of moisture under hot and wet conditions. If such an attack is prevented, then the corrosion of the steel wires can be avoided, thereby the steel cord adhesion level can be maintained under all aged conditions of the rubber tire. In this way the service life of the tire can be extended.
There are several existing approaches or methods employed to prevent or reduce the steel cords corrosion in rubber tires. These approaches or methods involve not only the rubber composition formulations but also the treatment of the steel cords. Although these approaches or methods may improve some desirable properties of rubber tires, the tire industry is constantly looking for alternative or better approaches to improve the properties of rubber tires including the corrosion resistance of steel cords and the steel cord adhesion between the steel cords and the rubber composition.
Therefore, there is a need for a new phenolic novolak resin that can be handled and used relatively safely in rubber composition formulations. Preferably, such resin can also improve the unaged, heat-aged and/or humidity-aged adhesion of the steel cords to cured rubber compositions.